Longitudinal transfer of rotated bunches in the CERN injectors

Bunch-to-bucket transfer between the CERN Proton Synchrotron and Super Proton Synchrotron for Large Hadron Collider–type beams is done via rotation of bunches in longitudinal phase space. For these rotated bunches, the dependence of beam transmission on bunch length had remained unexplained for a long time. Simulation-based optimization of the transfer was necessary to find the optimal rf settings for the rotation and to explain both earlier and new observations. Amongst others, we discuss why measured bunch profiles are insufficient to determine the best working point for operation. Crucially for future operation at higher intensities, we also show that the currently operational transmission can be maintained at a similar bunch length with a 40% larger longitudinal emittance, by using additional voltage from a spare cavity and optimal rotation timings.